1. Field of the Invention
This invention relates generally to the technology of mooring vessels such as oil tankers, floating barges, and the like, to a station where fluids may be flowed from a producing well or storage area to the vessel. In particular, the invention relates to a single point mooring system for vessels on a body of water.
2. Description of the Prior Art
The exploitation or operation of certain offshore oilfields as well as other industrial applications often requires the mooring of ships or floating vessels which are used for the storage of products and/or the transporting of fluids after extraction from a well.
Single point mooring systems have frequently been used in offshore locations for the loading and unloading of hydrocarbons or other flowable cargos into or out of marine vessels such as tankers, barges and the like. Many such systems have been developed and are now in use for both loading and unloading hydrocarbons at offshore locations. Examples of such conventional systems include the catenary anchor leg mooring (CALM) apparatus and the single anchor leg mooring (SALM) apparatus.
The catenary anchor leg mooring system (CALM) holds a floating buoy by an array or pattern of anchors attached by mooring chains or lines. The vessel which is to be loaded is attached to the floating buoy by suitable mooring ropes or a rigid mooring arm. In such a mooring system the position restoring mooring forces are provided by the horizontal force component in the mooring chains. However, in shallow water it has been difficult to arrange the mooring chains with sufficient slack to provide adequate resiliency or horizontal spring in the mooring system. In such instances, the horizontal movement of a moored vessel caused by external forces due to waves, winds and currents can cause sudden extremely large forces which may exceed the capacity of the mooring system to maintain the vessel in the hydrocarbon transferring mode.
Conventional single anchor leg mooring systems (SALM) have used one floating mooring buoy attached with a suitable chain or articulated arm to a lower base structure fixed to the sea bottom or floor. By submerging the buoy to a certain depth, a desired tension level is created in the anchor leg. The tension provides a constant restoring or horizontal urging force for resiliently mooring a floating vessel to the buoy. In shallow water the vessel position restoring capacity of the SALM mooring system is also reduced and not suitable for mooring large vessels. In general, the conventional SALM system has also not been particularly well suited for use in a location or region where large ice flows may contact or may be expected to impact the mooring buoy.
In view of the disadvantages of the prior art systems for mooring vessels in shallow ice laden waters, there has developed a need for a mooring system which may be completely submerged yet provide resiliency and restoring forces to a moored vessel while keeping it on station.
U.S. Pat. No. 4,351,260, issued in the name of Tuson et al., discloses a submerged single point mooring system in which a weight attached at the mid-point of articulated legs provides a resilient restoring force to the moored vesel. However, the system disclosed in the Tuson patent has a disadvantage in that articulated legs between a submerged tower and the vessel itself may be subject to mechanical instabilities and the like. For that reason, an object of this invention is to provide a fully submerged single point mooring system for a vessel floating on a body of water with an inherent resilient restoring force in response to waves, wind and current acting on the vessel.
It is a further object of this invention to provide a mooring system having a submerged unitary yoke rotatably attached to a fixed submerged base on the ocean floor and adapted to move in response to surge, sway, heave, yaw, pitch and roll motions of the moored vessel.
It is a further object of the invention to provide weighting mounting means on a connecting rigid yoke to secure weighting blocks providing a position restoring force to the vessel.
It is another object of the invention to provide a yoke suspension means with wear free articulations between the yoke and the vessel.